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BIG PHYSICS, BIG QUESTIONS –

Stellar archaeologist: We excavated the purest star

By Lisa Grossman

Astronomer Anna Frebel is part of a team that reported the discovery this week of a star that is almost as old as the universe – using techniques reminiscent of traditional archaeology. Lisa Grossman caught up with her at the Massachusetts Institute of Technology to quiz her about how that works.

You call yourself a stellar archaeologist. What does that mean?Real archaeologists dig in the dirt, in the desert – we dig in the sky. We try to find the oldest stars that are still there.

What do you mean by “still there”?They haven’t died yet. Their light is a few thousand years old because they are a few thousand light years away – we are looking at stars located in our Milky Way galaxy. This is a very different approach to looking at really distant galaxies whose light has travelled for billions of years – once that light arrives at a detector, it is really, really, old. So the stars in those galaxies may already be dead. That’s one way of looking into the past, but in our case we simply look for stars that are already very old – and that’s why we call it stellar archaeology.

So what is the deal with this star you have found, is it the oldest we have ever seen?It is definitely a superlative, but not in terms of age because, although we can estimate it, we cannot actually measure the age. It is a superlative in the sense that this star is the first we have found that we are very sure is of the second generation of stars in the universe. That is, of course, absolutely fantastic. This guy is nearly as old as the universe – it’s just amazing.

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Stars have generations?After the Big Bang, we started off with a universe that was just made from hydrogen and helium, the two lightest elements. There were trace amounts of lithium, but we don’t need to worry about this. Clouds of hydrogen and helium collapsed to form the first stars. All the other elements that exist and that we are made of – which are all heavier than hydrogen and helium – were created in stars and supernova explosions. These provide enough energy to fuse the nuclei of small elements and forge larger atoms.

So when a star explodes in a supernova, it seeds another generation of stars?Yes. And we look for stars with the least amount of elements heavier than hydrogen and helium. This indicates that not many prior generations of stars contributed elements to our stars’ birth gas cloud.

How can you tell what elements are in a star from so far away?We use telescopes to take what we call high-resolution spectra – meaning that we split the light up into a rainbow and throughout that rainbow there are colours missing. They have been eaten away by different elements absorbing pieces of light in the stellar atmosphere. From measuring and detecting how much light has been absorbed at which positions in the spectrum, we get a clue as to which kinds of elements are present in the star and how much of them. A picture may be worth 1000 words, but a spectrum is worth 1000 pictures.

How does the new star you found compare with, say, the sun?The sun is pretty metal-rich – a whole 1.4 per cent. Our guy has ten million times less than the sun – it is chemically the most pristine star ever found. That is why it is so special, and that’s what makes it a second-generation star.

Do you know approximately how old “your guy” is?Ages in astronomy are some of the hardest things to judge, and for individual stars there is no way of determining age unless there is a clock inside the star. Now that sounds a little silly at first, but we have actually found a few old stars containing radioactive elements like thorium and uranium, which decay over time at specific known rates and so can essentially serve as clocks. From comparisons with how much thorium and uranium was produced by the previous supernova, you can get an age-range. We have found that these kinds of chemically primitive stars that we are fishing out, they are something like 13 billion years old. Our new star is likely on this order.

Can your star tell us anything about the history of the universe?The calcium, magnesium and carbon and all these atoms that we see – in very small amounts – in our star must have been produced by a single, first generation star. It then died as a gigantic supernova explosion and all these newly created elements that it had made were ejected. Then our star formed.

So your star carries information about the first generation parent star?Yes. And, actually, it looks like that first star did not explode with the fullest power it could have. It probably tried, but it sort of failed. We can tell this because we don’t see any iron in our guy. Iron is heavier than the other elements in the first generation star like carbon, so it would have been in the core. The supernova probably went off, but there wasn’t enough power to yank out all the iron from the core and it just expelled its outer layers – this is what we see, in our guy.

Will we ever find first generation stars – the ones that exploded and gave your star all its elements?The issue is that the very first stars formed from this primordial gas which was just hydrogen and helium. This gas could not cool very well and it became kind of difficult to make small stars. Why is this important? Small stars – like the sun – have long lifetimes and the big, massive stars have really short lifetimes. So if there were no small stars among the first stars, then they have all exploded and they are all dead. We only have a chance of finding one of the first stars if more simulations can be conducted and they tell us that, by accident, the universe produced a low-mass first star. The next few years will hopefully clear this up.

Update&colon;This story has been updated for clarity since it was first published

Profile

Anna Frebel is an assistant professor in the astrophysics division of the department of physics at the Massachusetts Institute of Technology (MIT). She specialises in the early universe, the beginning of the chemical evolution and the formation of the first stars and galaxies.